Macroalgae such as kelp are important ecosystem engineers in the Polar Regions and potentially affected by freshening and ocean warming. The endemic Arctic kelp Laminaria solidungula might be particularly imperiled and become locally extinct from Arctic fjord systems in the future, since temperature increase is most pronounced in the Polar Regions. Additionally, increased temperatures cause glacier and sea ice melting and enhancing terrestrial run-off from snowfields, which eventually can result in hyposaline conditions in fjord systems. We conducted a multiple-stressor experiment at four temperatures (0, 5, 10, 15 °C) and two salinities (SA 25, 35) over 14 days to investigate the combined effects of increasing temperature and decreasing salinities on young L. solidungula sporophytes. The experiment was conducted with laboratory cultures (AWI culture number 3130) at Alfred Wegener Institute - Helmholtz Centre for Polar and Marine Research, Bremerhaven in February 2018. As physiological parameter, the maximum photosynthetic quantum yield of photosystem II (Fv/Fm; Imaging-PAM) was monitored every fourth day with an Imaging-PAM (Walz GmbH Mess- und Regeltechnik, Effeltrich, Germany). The total nitrogen, total carbon content was analyzed with an elemental analyzer and the C:N ratio calculated. Phlorotannins, mannitol as well as absolute pigment concentrations were analyzed using a HPLC and the deepoxydation state of the xanthophyll cycle (DPS) calculated.

Samples:laboratory cultures: gametophyte strain of Laminaria solidungula from Spitsbergen – as stock culture obtained from the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany; AWI culture number 3130 – not separated into male and female gametophytesTreatment details – two-factorial experiment:experimental set-up -> 4 temperatures, 2 salinities, 2L aerated Kautex bottles, ½ PES, 30 μmol m-2 s-1 PAR, 16:8-h-light:dark conditions, medium changed 2x/weekinitial -> 0°C, total salinity (SA) 35temperature acclimation -> within 1 weektreatments -> 0°C_SA25, 0°C_SA35, 5°C_SA25, 5°C_SA35, 10°C_SA25, 10°C_SA35, 15°C_SA25, 15°C_SA35 for 10 days